An organic light emitting device is an electric device that emits light through current by applied voltage. Tang et al. reported an organic light emitting device having a good characteristic in a treatise [Applied Physics Letters 51, p. 913, 1987]. Further, an organic light emitting device using a polymeric material while using a structure of the organic light emitting device disclosed in the treatise had been also developed.
The key point of the related art is to allow different organic layers to share roles for performing processes in which the organic light emitting device can emit light, such as electron injection, electron transport, formation of optical exciton and generation of light. Therefore, in recent years, as shown in FIG. 1, has been used an organic light emitting device including a positive electrode 7, a hole injection layer 6, a hole transport layer 5, an emission layer 4, an electron transport layer 3, an electron injection layer 2 and a negative electrode 1 or an organic light emitting device having a segmented structure constituted by more layers.
Have been made studies on doping the hole injection layer, the hole transport layer, the electron transport layer and the electron injection layer in the structure of the organic light emitting device with various different materials in order to improve mobility of the existing organic material (Publication of Japanese Patent Application No. 2000-196140, Treatise [Applied Physics Letters, 73, p. 729-731, 1998], Treatise [Applied Physics Letters, 72, pp. 2147-2149, 1998], U.S. Pat. No. 5,093,698, and International Unexamined Publication No. WO01/67825).
In the documents, a device having high efficiency is implemented by merely increasing the mobility of the charge transport layer or the charge injection layer through doping. For example, as disclosed in International Unexamined Publication No. WO01/67825, when the hole transport layer is P-doped with a stable acceptor type organic molecular material having a high molecular weight of 200 g/mol or higher (low doping concentration: 1:110˜10000), hole mobility is increased as compared with the opposite case. Similarly, it is disclosed that when the electron transport layer is n-doped with a stable donor type molecular having a high molecular mass, an effect similar to the above can be obtained.
Meanwhile, since the quantity of electrons that are injected into the emission layer from the electron transport layer smaller than the quantity of holes that are injected into the emission layer from the hole transport layer in the organic light emitting device at present, the efficiency of the device may be increased by decreasing the mobility of the hole transport layer (Applied Physics Letters, 86, 203507, 2005).
The document discloses an example in which a hole transport layer [N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), HOMO: −5.5 eV, LUMO: −2.4 eV] having a large energy band gap is doped with a hole injection layer [copper phthalocyanine (CuPC), HOMO: −5.1 eV, LUMO: −3 eV] having a small energy band gap. The increase in efficiency of the device results from increasing the efficiency by adjusting a ratio between the holes and the electrons injected into the emission layer by trapping the holes by using a highest occupied molecular orbital energy level of CuPC.
On the other hand, Patent (KR 1-0823443) disclosed the fact in that a high-efficiency organic light emitting device can be obtained when the hole injection and/or transport layer is formed by using a material having a LUMO energy level of −4 eV or lower in addition to a material having a HOMO energy level of −4 eV or lower originally used as a hole injection and/or transport material. However, in the related art, it is difficult to achieve low operating voltage when the thickness of the organic layer is large.
An effect resulting from decreasing the operating voltage of the organic light emitting device may increase energy efficiency and has been known as a matter which is essential to form an optical resonant structure particularly required in actual application of the organic light emitting device. The known formation of the optical resonant structure can be classified into (A) application of an internal resonant structure [ref(Journal of applied physics v97 093102)], (B) application of an MIM resonant structure [ref(Journal of applied physics v97 103112)], and (C) application of an external resonant structure [ref(Journal of applied physics v93 p19), (Journal of applied physics v86 p2407), and (Journal of applied physics v80 p6954)]. Among them, in the case of (A) and (B), the present invention has been made in an effort to increase energy efficiency of the entire organic light emitting device and partially solve a limitation related to voltage generated in forming the optical resonant structure required in actual application by providing a technique of decreasing the operating voltage of the organic light emitting device.